Solid freeform fabrication processes are increasingly used for producing freeform, complex geometry components. In particular, selective laser sintering is useful for creating three dimensional objects directly from digital data (e.g., computer-aided-design (CAD) databases, medical scan imaging files, etc.) in an automated fashion. An object is created through a sequential fusion buildup of cross sections of the finished part from a starting powder. The powder is fused with a scanning laser beam one layer at a time, and each scanned layer corresponds to a cross section of the final object's mathematically sectioned digital data model.
Detailed descriptions of selective laser sintering technology are provided in U.S. Pat. Nos. 4,863,538; 4,944,817; and 5,132,143, all assigned to the Board of Regents, The University of Texas System, and in U.S. Pat. No. 4,247,508 to Housholder.
Laser sintering is a thermally based process. The sources of thermal energy are typically heaters for the part bed, cylinder heaters to preheat the powder in powder feed cylinders, and the laser. The laser is typically a CO2 laser that scans the fresh powder layer to selectively fuse powder particles in the desired areas. The powder is normally kept near the fusion temperature of the particular powder so that the added heat required by the laser to fuse the part is minimized. The part-cake (essentially comprising a block of fused and unfused powder) remaining after the completed layerwise build process is hot.
Before the laser sintering process can be restarted to build a new part or series of parts, the part-cake is cooled down to a handling temperature safe for the personnel handling the apparatus and acceptable for removing the built parts without damage to the parts. Powder materials commonly used in laser sintering have comparatively poor thermal conductivity and so can take a long time to cool down to workable temperatures, particularly at the interior portions of the part-cake. The time required for cool down of the part-cake is further increased as larger build areas are use (i.e., one or more of the X, Y, or Z dimension of the part cake is increased). The increased cool-down time reduces efficiency of the laser sintering process by increasing the residence time for each part build session, thereby decreasing throughput of the laser sintering system.
The cool down portion of the laser sintering process can be detrimental to the built parts if the cooling is non-uniform throughout the part-cake causing uncontrolled thermal gradients therein. Non-uniform cooling can lead to unacceptable geometric distortions of the built parts in the part-cake, as well as inconsistent mechanical properties of the built parts.